The recent growth in the demand for broadband services has resulted in a pressing need for increased capacity on existing communication channels. The increased bandwidth of optical fiber communication links are still often insufficient to cope with this demand without utilizing the ability of these fibers to carry large numbers of individual communication channels, where each channel is identified by the particular wavelength of the light. This technique is known as dense wavelength division multiplexing (DWDM).
Linear optical fiber communication technologies are essentially based on the same principles as radio frequency systems. Fiber communication systems, however, are fundamentally different, because they make positive use of the inherent optical fiber property of nonlinearity. Rapid progress in nonlinear lightwave communications is stimulated by increasing demand for telecommunications services. Practical and research interest is directed mostly toward two main goals: development of effective high capacity long-haul transmission systems and the upgrade of existing terrestrial fiber networks.
There are three major factors that cause optical signal degradation and distortion in long-haul high bit-rates fiber communication systems:                Loss due to absorption in the fiber;        Group-velocity dispersion (GVD); and        Optical nonlinearity.        
Signal power attenuation due to absorption can be compensated using amplifiers, although recovery is not complete since amplified spontaneous emission noise is added to the signal and degrades the signal-to-noise ratio. Revolutionary developments in nonlinear lightwave communications have been triggered by the development and deployment of optical amplifiers providing periodic amplification of optical signals. The portion of the optical link between each amplifier is commonly known as a span, where each successive amplifier at a node position amplifies the optical signals that have degraded by propagation through the previous span.
Until the invention of the erbium-doped fiber amplifier (EDFA), optical signals were regenerated electronically to overcome the attenuation in the silica fiber. Electronic regenerators have two important drawbacks: they are expensive and they limit system performance, because each regenerator can operate at only one predetermined bit-rate, in one data modulation format and at one operating wavelength.
Because the EDFA has many important advantages (such as large bandwidth, high gain, simplicity and others) over optoelectronic regenerators, they quickly became the amplifier of choice in communication systems. As a result, fiber loss is no longer a major limitation in optical fiber transmission and the performance of optical amplifier systems is then limited by CD and nonlinearity. Note that whereas a regenerator re-creates a perfect digital output signal, the fiber amplifier uses whatever it receives. Therefore, dispersive pulse broadening and other degrading effects are accumulated along a fiber line.
There are two principal approaches to overcome these limitations: in the first instance, both the chromatic dispersion (CD) and nonlinearity are considered to be detrimental factors, however, due to short fiber lengths and small optical intensities in the fiber, the nonlinear effects are only small and therefore are ignored. This is known as a linear approach. In the second approach, the nonlinear and dispersive effects are recognized under certain conditions as being reciprocal effects that can be counterbalanced by appropriate design of the link architecture. Such systems are called nonlinear since the nonlinear effects that are detrimental in the linear systems are used to improve transmission characteristics of the optical communication system.
As the demands on the capacity of the communications links increases further, the modulating frequency of the optical signals must increase. Current standard systems operate at data transmission rates of 10 Gbits per second per channel so that, in a DWDM network containing 50 channels, the total transmission capacity of the link is 500 Gbits/second. The next evolution in communications links requires this to increase and the next goal is to have reliable links operating at 40 Gbits/s per channel. At this data rate, electronic components used in repeaters and at the receiving end of the link struggle to keep pace with the amount of data being transmitted. Thus the demand is to be able to transmit the information through the link in wholly optical form, without any conversion to an electronic signal. This will certainly be required for the further generations of optical communications links where data rates of 100 Gbits/s per channel and greater are predicted.
A light pulse is an electromagnetic wave packet built from a continuum of elementary optical carriers oscillating at different frequencies. In other words, any optical wave-packet contains a range of frequency components. Since any optical fiber is a dispersive medium, each of these spectral components travel at different group velocities, causing the pulse energy to spread over time as the pulse propagates through the medium. Fiber GVD is measured either in units of picoseconds squared per kilometer (ps2/km) or picoseconds per kilometer per nanometer (ps/km.nm). Roughly speaking, a pulse with the bandwidth 1 nm spreads by corresponding number of ps over 1 km. Dispersion can be positive where low frequencies travel at a higher speed than high frequencies or negative where high frequencies propagate at a higher speed than low frequencies. The dispersion of standard single mode fiber (SMF) is positive (also called normal) for wavelengths shorter than about 1300 nm and negative (anomalous) for wavelengths longer than 1300 nm. SMF has dispersion of about 20 ps2/km at wavelength 1550 nm. Corresponding dispersive spreading of a 10 ps pulse in SMF after 125 km is about 50 ps or, in other words, 5 times its original width. Such a large spreading can lead to overlapping in the time domain of neighboring bits and consequently to degradation of the information signal.
Linear signal distortion caused by the GVD in fiber transmission systems can be almost suppressed by the dispersion compensation or dispersion mapping technique. Optimization of the system performance in the case of a linear transmission requires minimization of the CD of the optical communications link. This can be achieved by operating close to the zero dispersion point or/and additional compensation of the accumulated dispersion. The idea to use a compensating fiber to overcome dispersion of the transmission fiber was proposed in 1980. In the low power linear regime where the response of the fiber is linear, compensation of dispersion aims to prevent dispersive broadening of the signal in the transmission fiber by the compression in the compensating fiber.
In linear systems dispersive broadening can largely be eliminated by dispersion compensation. However, the nonlinear effects can still be the primary reason for signal degradation especially in long-haul transmission systems. The response of the optical medium is not exclusively linear. The fiber refractive index instantaneously increases by an amount proportional to the optical power. This phenomenon is known as the optical Kerr effect. Modulation of the optical power leads to the corresponding modulation of the index. For instance, a high power light pulse increases the refraction index with corresponding change of the phase of the propagating pulse. This effect is known as self-phase modulation (SPM).
SPM involves an interaction of an optical pulse with itself. SPM of an optical pulse does not cause any degradation of other bit-pulses of optical signals of different frequencies propagating through the link and so does not significantly contribute to the interchannel cross-talk degradation. When the dispersion map of an optical link is designed, the dispersive effects of the individual components that make up each span in the map are considered and selected to optimize the SPM to acceptable levels across the whole link. SPM can even be beneficial when the wavelength of the signal pulses falls in the anomalous region of the optical fiber characteristics as it can partially compensate effects of CD in the pulse by delaying the ‘fast’ spectral components relative the ‘slow’ components.
The dispersion compensation technique has been used relatively successfully both in long-haul communication systems and in the existing terrestrial optical links, most of which are based on standard telecommunication fiber with large dispersion in the optical window around 1550 nm. The basic optical-pulse equalizing system consists of a transmission fiber, which is typically standard SMF already existing in the installed link, and one or more lengths of equalizing fiber possessing a large dispersion coefficient of opposite sign known as dispersion compensating fiber (DCF).
Prior art dispersion management schemes such as those disclosed in U.S. Pat. No. 6,832,051 to Lu et al and U.S. Pat. No. 6,417,945 to Ishikawa et al, while being effective for single channel fiber communication systems, have at least one shortcoming with regard to multi-channel systems. Specifically, complete correction of dispersion in all channels at the end of the system is not easily accomplished, primarily because the dispersion slope in the compensating fibers typically cannot meet the two requirements of being both high in magnitude and negative in sign. Thus, fibers with high negative dispersion and high negative slope are difficult to manufacture and therefore expensive. Small variations in fibers with these properties typically lead to large changes of other properties of the fiber, and hence are typically not reliably manufacturable. Also, there is a large installed base of SMF fiber, and even if DCF were easier to manufacture, replacing of the existing outside cable plant would be very costly.
A further disadvantage of using dispersion-compensated fiber (DCF) includes the added loss associated with the splicing to the initial fiber length and the increased fiber span. Thus, the amplifier stage in each span of the communications link must also compensate for this additional loss. Additionally, the nonlinear effects may degrade the signal over the long length of the fiber if the signal is of sufficient intensity.
A dispersion management scheme that has been applied to a multiple channel transmission system is disclosed in U.S. Pat. No. 6,659,614 to Katayama. This patent discloses separating the individual wavelength channels in the signal and directing them onto a single deformable mirror. The mirror is then deformed into a substantially parabolic shape to correct for large-scale optical dispersion across the whole optical transmission window of the communications link. This patent, however, does not teach individual control over each channel independently. As such the ability to control different transmission impairments is limited.
U.S. Patent Application 2003/0170939 to Moon et al discloses a chromatic dispersion compensation device based on a micromirror array. Moon provides a system whereby each of the dispersed wavelength channels is incident on a plurality of micromirror “pixels” which are switched between one of two positions to delay part of the light incident on the array by a predetermined amount and partially compensate for chromatic dispersion in a wavelength channel through providing a number of quantized phase levels.
Disclosures by Katayama or Moon et al. both rely on mechanical adjustments of mirrors which is disadvantageous in a high reliability Telecommunications environment and furthermore neither teach the advantageous use of smooth adjustment of phase within a channel to vary group delay in combination with control of the relative group delay or relative phase between wavelength channels or the simultaneous compensation of a variety sources of optical signal degeneration in an optical communications link.
Other techniques used for dispersion compensation at each node in the dispersion map include:                Multiple-Cavity Etalons, which includes Gires-Toumois Interferometers (GTIs), such as those disclosed in U.S. Pat. Nos. 6,768,874, 6,748,140 or 6,654,564, and by D. Moss et al, Presentation TuD1 entitled “Tunable Dispersion Compensation at 10 Gbit/s and 40 Gbit/s Using Multicavity All-pass Etalons”, Conference on Optical Fiber Communication (OFC) 2003, Atlanta Ga., USA; or        Chirped Fiber Bragg Gratings (CBFGs) such as those disclosed in U.S. Pat. Nos. 6,847,763 or 6,807,340.        
The GTI is a bulk optics element that can be configured to operate in either reflective or transmissive mode and compensates for dispersion through wave interference mechanisms. Since it is a bulk optic element, it can be configured to provide a high compensating dispersion through a relatively short propagation distance. Additionally, it can be constructed such that the dispersion at a given center wavelength is tunable. The most practical mode in which a GTI is used is in reflection where the back face of the resonator is 100% reflective, however these suffer from high insertion losses due to the need to isolate the back propagating signal. This can be partially compensated by the use of optical circulators, however, GTIs suffer from the fundamental disadvantage that the effective bandwidth decreases as the dispersion is increased.
In contrast, the CFBG can be designed to simultaneously have a high dispersion and large bandwidth. It does, however, still operate in the reflective mode and so still suffers from the insertion losses and the need for optical circulators within the span. Fiber Bragg gratings can also induce dispersion ripple, which leads to undesirable distortion of the optical signals.
All these existing dispersion compensating techniques, however, possess a common problem. Since the amount of dispersion that each optical channel experiences depend on the frequency of the channel, it is extremely difficult to completely return all the channels to a common dispersion value at each node. This effect is depicted in FIG. 1 which shows the (exaggerated) GVD over two spans in a dispersion managed optical communications link. A plurality of optical channels are shown ω1 to ω4 (where ω1<ω4). As the optical signals propagate through the SMF 1 of Span 1, the signals of higher frequency experience a greater GVD. When the signals reach the length of DCF 2, the highly negative dispersion partially reduces the GVD. The problem lies with the difficulty in correcting for the GVD across all the optical channels and so the signals enter the next span of the link with a GVD mismatch indicated by the 3. As the signals pass through multiple spans of the link, the GVD mismatch increases as can be seen at the end 4 of the length of DCF fiber 5.
The total GVD mismatch accumulated through all the spans of the link need to be compensated at the final receiver stage of the system. At present, this is achieved by separating each of the DWDM channels on the link and feeding them into separate electronic dispersion compensators. Again, these electronic systems are adequate for current data rates of 10 Gbits/s, but struggle at increased data rates of 40 Gbits/s and upward. Thus, the need for all optical dispersion and GVD compensators for further advances in optical communications systems is paramount.
As has been discussed, the detrimental effects of the optical fiber link in current systems are managed by modeling of the whole optical communications link and inserting lengths of fiber into the link at regular intervals to amplify the pulse to counter the effects of optical loss in the fiber in the time domain, and optical dispersion in the frequency domain. This is a valid method, provided that the optical signals propagating through the link experience all the spans as designed.
In reconfigurable networks, where optical signals can be added and dropped to the link at any number of stages along its length by reconfigurable add/drop multiplexers (ROADMs), the dispersion map is less valid. The optical signals have not propagated through each stage to get the benefit of the carefully designed dispersion and GVD properties of the link as a whole. Extra care must be given to these signals to determine their properties once they have been dropped from the link and compensated to avoid loss of the information they contain.